Isoprenoids, also referred to as terpenoids, constitute the largest and most diverse group of naturally occurring organic chemicals, with more than 25,000 different member compounds. The smallest isoprenoid is isoprene (C5H8) a volatile five-carbon hydrocarbon with considerable commercial potential, as it is feedstock in the synthetic chemistry industry, where it is used to manufacture products ranging from rubber to adhesives and perfumes. There is also potential for isoprene to be developed as a renewable biofuel, where oligomerization of isoprene units may generate second order fuel molecules, suitable for use as supplements of gasoline, jet fuel, and diesel. Currently, the industrial supply of isoprene is limited to petrochemical sources. Sustained availability of these resources is being questioned by the increasing global demand for energy and synthetic chemistry feedstock. Accordingly, there is a need to develop methods for the renewable production, sequestration and trapping of volatile isoprene hydrocarbons in processes that can be scaled-up for industrial application.
A variety of herbaceous, deciduous and conifer plants naturally produce isoprene via the process of leaf photosynthesis. Heat-stress of the plant is a prerequisite for the induction of isoprene synthase (IspS) gene expression and enzymatic activity (Sasaki et al. 2007; Sharkey et al., 2008). As a small hydrophobic and volatile molecule, isoprene easily passes though chloroplast and cellular membranes and is released into the atmosphere though the stomata of leaves (Sharkey and Yeh, 2001; Sharkey et al., 2008).
Given the volatile nature of isoprene (boiling point=34.1° C.), it is impractical to attempt to harvest this hydrocarbon from herbaceous, deciduous or conifer plants. Microorganisms, however, can be cultivated in fully enclosed bioreactors that offer advantages in product containment and sequestration. Although some microorganisms naturally produce isoprene (Kuzma et al., 1995) the quantities are small. Over-expression of a plant IspS gene in a photosynthetic or non-photosynthetic microorganism can effectively endow the property of isoprene production from the cell's own metabolism (Lindberg et al., 2010; Whited et al., 2010; Miller et al., 2001). Isoprene synthase (IspS) has been expressed in photosynthetic cyanobacteria and microalgae (see also, Lindberg et al. 2010, WO/2008/003078 and WO2008/137092).
There is also a need for additional methods for the renewable production, sequestration and trapping of other volatile compounds such as alcohols and aldehydes.